Deposits of minerals, ores, salts, brown coal and also pit coal are generally very extensive, and in cases where they are located not too deep below the earth's surface it is customary, for cost reasons, to work these deposits by open-cast working, even if relatively thick topsoil has to be removed previously in order to reach the actual deposit.
In Australia and also in South Africa, for example, enormous coal deposits are present close beneath the earth's surface, extending over many square kilometers.
So-called "Surface Miners" have been developed for working these deposits. These are apparatuses with a self-propelled chassis, with a cutting cylinder mounted with vertical adjustability thereon for cutting away the deposit to a specific depth, and with a loading apparatus for the material cut away.
These machines travel across the deposit and cut the deposit material away to a specific depth and simultaneously load the material, which is produced in comminuted form, by means of the loading apparatus onto transport vehicles which travel at the same speed behind or beside the apparatus and thus ensure continuous transport away.
Although the deposits frequently exhibit a relatively great thickness of the strata, nevertheless these strata do not always extend precisely parallel to the surface. Distortions and shifts in the earth's crust in the course of thousands of years have on the contrary caused the deposits which are to be worked to disappear in the earth's surface again at certain points and then suddenly come to light once more.
Furthermore, the mineral or coal strata to be worked frequently alternate with barren topsoils, which has the consequence that in the case of the open-cast working process by means of the "Surface Miners" mentioned above, not only the required deposit material, coal for example, but also frequently barren rock is transported. Even when the working apparatus is still definitely traveling over coal or mineral strata, it may nevertheless occur that the thickness of these strata at certain points does not correspond to the cutting depth, so that only a fraction of the material produced is actually coal or another required deposit material, and a large quantity of barren rock is produced additionally because the cutting cylinder of the working apparatus has cut too deep at certain points.
This proves an extraordinary disadvantage in practice, because the material thus produced by the "Surface Miners" must be separated again from the barren rock.